Optical lines such as a passive optical network (PON) are becoming more common in place of metal lines such as asymmetric digital subscriber line (ADSL) as communication lines for subscriber loops accompanying the increase in demand in communications. An optical terminal station device and a plurality of optical terminal devices that constitute PON represent a form of access system optical network connected through an optic coupler for splitting light as disclosed in, for example, Japanese Laid-open Patent Publication No. 2005-269239.
An optical terminal station device is called optical line termination (OLT) and is a communication device installed in a building of an optical communication service provider. The optical terminal device is called an optical network unit (ONU) and is a communication device installed in the home of a subscriber of the optical communication service. The transmission direction from the ONU to the OLT is defined as the uplink direction, and the transmission direction from the OLT to the ONU is defined as the downlink direction.
The optic coupler configures an optical network having a star topology by connecting one OLT with, for example, thirty-two ONU devices. One benefit of the PON is that the number of optical fiber lines (transmission lines) installed in the building and in the homes is reduced due to the star topology.
Conversely, one problem with PON relates to the bandwidth efficiency of the uplink signals transmitted from the ONU to the OLT. Since the optic coupler and the OLT are connected by one optical fiber line, uplink signals are previously attributed with individual transmission timings from the OLT to each ONU in order to avoid collisions. As a result, the uplink signals from the ONUs are transmitted to the OLT using time-division multiplexing (TDM).
However, a certain amount of guard time is provided in the headers of the uplink signals since the arrival time of an uplink signal varies due to environmental factors such as temperature and the like. The guard time reduces bandwidth efficiency since uplink signal bandwidth is consumed.
Another problem with PON relates to transmission distance. The ONU may not be provided with an optical transmitter capable of long distance transmission due to cost restraints. While lengthening the transmission distance by providing an optical amplifier in the transmission line may be considered, surges in the optical amplifier may occur due to burst signals such that the uplink signals from the ONU are not transmitted in a stable manner.
In contrast, the above problem can be resolved by adopting frequency multiplex transmission such as orthogonal frequency-division multiplexing (OFDM) in the access system optical network. In this case, collisions are avoided and time division multiplexing does not have to be used since the uplink signals from the ONUs can be conveyed at the same time using subcarriers with different frequencies. Moreover, the uplink signals from the ONUs may allow the transmission distance to be lengthened since burst signals do not have to be used.
However, when frequency multiplex transmission is adopted, beat noise may occur between uplink signals due to the relationship between the oscillation wavelength of the light of the uplink signals and the subcarrier frequency (modulation frequency) since the uplink signals from the ONUs are carried by subcarriers with different frequencies. Specifically, since the frequency spectra of the uplink signals overlap, different frequencies are generated and transmission errors may occur.